A major trouble to be overcome in the conventional types of subsurface oilfield pumps is technically known as a "gas lock", and happens when the incoming pressure in the tubing is kept up by the orifice outlet valve, or travelling valve, on the upstroke of the piston, and by the orifice inlet valve, or standing valve, on the downstroke of the piston. This downstroke of the travelling valve gives rise to pressure within the fluid, between the traveling and standing valves, and causes the traveling valve to open thus enabling fluid to pass through the traveling valve or orifice outlet valve. However, when operating in a well that is producing both oil and gas at the same time, the chamber placed between the travelling valve and the standing valve is often filled with gas, and because of the compressibility of the latter, the downstroke of the travelling valve may not create enough pressure in the chamber below the aforesaid valve to offset the pressure of the column of fluid standing above the valve, which means that therefore the travelling valve remains closed throughout the downstroke. Thus, the gas between the standing valve and the travelling valve only compresses and expands at every stroke of the piston, which leads to the pump operating defect known as "gas lock", a state of affairs which may go on indefinitely.
PI 8501271 of Mar. 19, 1985 concerns a system meant to provide an answer to the troubles referred to above, and consists of an elongated housing with upper and lower ends, a first valve fitted into the bottom end of the housing, a part to drive the travelling valve fitted in the upper end of the housing and placed so as to slide lengthways in relation to the housing, a rotating travelling valve fitted between the first valve and the part that drives the travelling valve, the travelling valve having upper and lower ends and a sealing surface against either end, a piston to compress fluids, lying between the first valve and the part that drives the travelling valve, and a means to rotate the travelling valve around its lengthwise axis, such rotating means being connected to the part that drives the travelling valve, and the travelling valve itself, whereby the lengthwise movement of the part that drives the travelling valve causes the travelling valve to rotate. The first valve is worked by changes in the pressure of the fluid, which take place inside the housing while the travelling valve and the part that works the travelling valve operate mechanically.
As regards performance in the foregoing system, note that gas locks, hydraulic chock and sealing defects caused by vibration of pump piston are avoided, though the same does not apply to wear, since there is no way of ensuring that particles of matter may not get into the travelling valve assembly, and if this does happen there may be serious trouble, not only as regards wear but also locking and breaking thereof, for if particles store in the joints this may be enough to bring about locking, and since operation is mechanical, considerable force is exerted upon the helical part, which is the most fragile in the system.
Positive displacement action pumps are also used. Throughout discharge the standing valve remains closed and the piston moves from its furthest position to its closest position as regards the standing valve. When this happens the piston tends to stay in the same place owing to the effect of friction between it and the pump body, as well as because of the effect of the counter-pressure created between the travelling and the standing valves, as the pump moves towards the standing valve. At same time all the weight of the pump rods are bearing directly on the plug, forcing it to be pushed off the valve seat. This forced opening promptly prevents any gas or vapour lock from taking place.
When the valve opens the distance between the seat and the plug is limited by a stem that joins the plug to the connection. This distance is calculated beforehand in such a way as to enable the fluid to flow forward of the opening under less resistance.
As soon as the piston gets to the point closest to the standing valve it acts in the opposite direction, into its initial suction stroke. Again friction between the piston and the pump body tends to keep the piston back until the plug seals against its seat. This takes place when the relative speed of the fluids at either side of the valve is null, therefore the effect of any erosion upon sealing surfaces is considerably less.
When the travelling valve is closed the pressure between it and the standing valve reduces as the piston moves off from the standing valve, until it becomes lower than the pressure in the reservoir. When this happens the standing valve opens and lets fluid from the reservoir into the pump body. Finally when the piston gets to its point furthest away from the standing valve it moves in the opposite direction and the pumping cycle is repeated.
However a disadvantage of the aforesaid system is that particles of matter store and prevent operation from being ideal, since the relative movement of any fluid bearing particles of sand in suspension erodes the sealing portions of ball or piston valves (particularly in the case of the travelling valve concerned), because of rubbing by particles of silica in any kind of sand.
Another disadvantage is that it is difficult to make use of existing pistons, since not just any kind of piston may be used, and also there is the end cost of the equipment to consider.